If everything was working fine before the window tint job I wonder if they could have knocked a vacuum line off in one of the doors. Did they pull the door panels off when they did the tint? You could block off the vacuum line going to the locking system and see if it holds a vacuum then. You will be looking for a little yellow canister in the engine compartment by the firewall on the drivers side. It is just between the master cylinder and where the accelerator linkage comes through the firewall and has one hose going in from the engine bay side and two going out towards the firewall. Pull of the hose on the engine side and plug it with a golf tee and see what happens.
ben

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1985 300D 170k